Electrode for compact type electrical discharge devices



D. L. HILD'ER `Fam. 26, 1954 ELECTRODE FOR COMPACT TYPE ELECTRICAL DISCHARGE DEVICES Filed Jan. ll, 1951 ,vf .J..,v\\

INVENTOR.

BY WW1.

ATTORNEY Patented Jan. 26,` 1954 ELECTRCDE Eon COMPACT TYPE n CAL DISCHARGE DEVICES David L. Hilder, Allentown, Pa., assignor Ato Hanovia Chemical and Manufacturing` Company, Newark, N. J.,4 a corporation of New Jersey Application January 11, 1951, serial No. 205,550'

The present invention deals with a compact Atype electrical discharge device and more particularly with an electrode therefor.

The compact type electrical discharge lamp is distinguished from the tubular and capillary type discharge lamps in that the lamp envelope is of a substantially spherical configuration and the'electrodes contained therein are spaced to provide a discharge path of from a few millimeters tofabo'ut two centimeters in length. 'Ihe electrodes in lamps of this type are subjected to very high current densities and, especially during the starting of the discharge when the electrodes are cold, to a high evaporation rate of the metal forming the electrodes, which blackens the walls of the lamp envelopes, e. g'. light transmissive envelopes of fused silica, fused quartz, or other glasses of high melting point, and thereby decreases the light output of the lamps. The

rate at which the electrodes evaporate is the de- Vtermining factor of the useful life ofthe lamp.

Tungsten'is generally used as the electrode material. The electron emissivity of tungsten increases enormously with temperature, being of the order of one hundred billion times greater at near the melting point than at about 25 centigrade, and the temperatures developed inthe discharge chamber of compact type lamps employing su-ch electrodes are exceedingly high, e. g.

between about 5,000 and 11,000 Kelvin. How- :n`

ever, tungsten melts at 3,643 Kelvin and the electrodes will melt unless employed in very specific forms to dissipate zby radiation and conduction the heat received from the discharge.

Usually, the electrodes used in compact type lamps are massive in structure-in comparison with the electrodes of other type lamps having envelopes of comparable size in order to provide ample surface for radiation because radiation is a surface phenomenon. Massive electrodes have certain undesirable features in that the initial emissivity is low and sputtering of the vtungsten metal is likely to occur,` the extent of sputtering being dependent in part on the rate at which the metal warms. For the same current, a massive electrode warms more slowly than a substantially smaller-sized electrode and, therefore, sputters to a greater extent and more rapidly blackens the envelope than the smaller electrode during the heating up period.

Electrodes of substantially less mass than the aforesaid massive electrode will heat faster and sputter less but will evaporate more rapidly after they have been heated and blacken the envelope more rapidly after the heating up period. More- 1. Claim. (Cl. 313-39) 2 over, such smaller-sized electrodes, due -totne rate of evaporation ofthe tungsten aftervthe heating up period,V causes the lampemploying `such electrodes to have a shorter useful life than lamps employing the more massive electrodes.

Therefore, specific forms of electrodes for compact type lampsfare a compromise between rate of Vinitial sputtering action and subsequent evaporation. Y Y' fw Itis an object of the present invention vto provide an improved electrode for compactt'ype lamps.` It is another object of the presentinvention to provide an electrode which combines the advantages of massive and substantially smaller-sized electrodes. It is a further object of the present invention to provide a compact type electrical dis-charge'lamp which employs the iniproved electrodes `for a longer useful life' of such lamps. It is a still further object of the present invention to provide a compact type lamp and electrodes therefor having a'combination of embodiments which cooperate to provide a superior compact type lamp. VOther objects and advantages of the present invention will become apparent from the descriptionhereinafter "following and drawings forming part hereof, in which:

' ,Figure 1 illustrates partly in elevation and partly in section a major portion of a compact type lamp Vaccording to the present'invention, Figure 2 illustrates an enlarged view of the salient features according to the presentinv'ention, and I" Figure 3 illustrates a partial view of Figure 2 including an improvement of the presentinvention.

'Ihe present invention is concerned with high pressure and super high pressure rare gas or rare gas and metal vapor compact type discharge lamps having light transmissive substantially spherical envelopes of fused quartz kand the like glasses, and an improved electrode structure which avoids in a large measure the inadequacies and disadvantages of electrodes :heretofore 'employed in compa-cttype lamps. f Referring to Figure 1, the compact type lam of this invention comprises a substantially spherical vitreous e. g. fused quartz, envelope I dening a discharge chamber and having tubular fused quartz extensions, e. g. tubes 2 and3 projecting outwardly therefrom preferablyv oppo- Asitely of each other, which support a pair of spaced electrodes 4 and Sfhereinafter referred to as main electrodes. The -main electrodes lare spaced from each other in the discharge chamber to provide 'a discharge path-of from a fewmilli.

envelope. portion' oi the electrode outside the discharge type structure.

meters, e. g. 5 millimeters, to about two centimeters in length. The discharge chamber contains an ionizable atmosphere of a rare gas at at least one atmosphere pressure at about 25 C., or a rare gas and a vaporizable metal, e. g. mercury, infan-lamountsuiiicientfwhen vaporized to provide gas-vapor lil-ling of at least' one atmosphere total pressure at about 25 C.

The improved electrode structure of the pres ent invention comprises the aforesaid pair of spaced main electrodes as portionsfofrods'v and 5 upon each of which is mounted anarrange-r ment of auxiliary electrodes. Each main! electrode consists of a rod of substantial mass-,e. g. a rod having a diameter of;,approxi-.u1`atelyf 02004 inch per ampere of normal current rating for the electrode, said current being in, the order of at least 5 amperes for a compacttype lamp. The length of each main electrode or rod is amessential feature in that it is at least three times the y,lengtlti of the inside. radius. of the spherical envelopefand amajor-portion-oi the redis encased or sealed and supported by a vitreous. or glass tube or extension,ve..g..tube 2, anda minor portion of the. rodV extends intov the discharge chamber-, said v major portion acting to conduct heatv from said minor portion and said glass tube assistingin the conduction ofheat away from the dischargej and electrode and thereby cooling the electrode by conduction-and convection. It is apparent that the: tubes '1.1V andr3, sincethey seal-in major portionsof theelectrodes and E, are themselves of substantiallength thereby allowing suicientcon 'duction and convection. to prevent at least the ends ofthe rods A and 5` outside the'discharge chamber from incandescing. If the length of the .electrode sealed into the tube 2 istoo short relativeto that portion extending into the discharge chambenthe arcingy ends of the electrode will overheat, possibly meltfand in such case will .evaporate f Accordingly, if the length of themajor chamber is at least 2/3. thelength of the electr-ode, preferably from about 2/3 to the length, suf- 'cient conductionandA convection are main-tained to prevent the above mentioned detrimental conditions. Howe-ven in regard to the lower limits of lamp-temperature and the diameter of thefelectrede, in some instances it isY sufcient'that a major Yportion of the electrode isl outside the discharge chamber.

VrElieY main electrodes are preferably composed of .commercial tungsten withoutthe addition: of;V activators since the latter are not satisfactory because they evaporate toofreadily atthe operating temperature of the electrode, which at ilts arcing end isat about 3400" Kelvin. It is apparent that the'major V.por-.tion of the main electrode, is not Isealed; vaouumtightly. Therefore, a.V vacuum tight seal. is. connected tothe end ofV the ,main electrode which is outsidethedischarge chamber. This vacuum'tightV seal is.- sealed by said tubev 4 as is the main electrode and is electrically conductiv-eand of sufficient capacity to carry thenor- 4mal.currentrating for theelectrode.

For exam-ple,v the vacuuinftightseal may bein the v-f-orrn of'Y a substantially thin stripor Vsleeve- '5 las illustrated ande-composedof molybdenum vand the like materials known.tothe arttoconstitute suitable materials .for such seals.

TheV minor portion of bothl main electrodes, ,whichv extend into. the-dischargechamber, have mounted thereon an arrangement of. first auxelectrodes 'I `and & whichassist the start of rapidly and blacken the walls of the 4 the discharge and second auxiliary electrodes 9 and I0 which warm rapidly to take up the discharge from electrodes 'I and 8 and sustains the discharge until the ends I I and I2 are heated sufficiently to take up the discharge to establish a fixed arc. The whole starting, procedure including'the establishment ofthe xed arc occurs in a fraction of a second when a lamp employing such an electrode structure is connected to a suitable power supply.

IiiiordertoJcarr-y out the objectives hereinbefore setiforthf and to enable the electrodes to operate as abovefdescribed, it is essential that the main electrode andl the-` auxiliary electrodes supported thereon-form an electrode structure which comprises a combinatibn of the said electrodes in a substantially critical. relationship to each other. 'iiince'tlie minor portions of main electrodes 4 and. 5 particularly the ends II and I2, are relatively massive they canoperate and sustain a discharge arc without evaporating v suiciently to render. the lamp susceptible t'o a. short, useful life. However, the sec'ond auxiliary electrodes, order to .operate asvdescribedmust beY of less mass, than the minorportionsof'the mainelectrodesin. order to neat' up rapidly enough to I'IaleupV the discharge from the iirst` auxiliary electrodes almost. instant,- ly, andimust notbe. positioned;too near to the fixed arc sustained. by the main electrodes to prevent the evaporation thereof. 'Iliiereforethe'second auxiliary electrodesI il and III areV support/'eden the minor portions of they main electrodes but spaced' iromthe arcing ends- [II and I2'. These second auxiliary electrodesare preferably spaced from the ends II- and |21 for a distance relative t'o the Adierencein mass betweenfthe two said. elec,- trodes. For example, as a preferred embodiment the second' auxiliary electrodes. may comprise tungsten coils of about 5 to 7 turns Aeaohand the diameter of.. thel Wire forming said coils. being about ve-eigliths the diameter oi. the mainelectrodeslsaid! coils being. spaced, from .theends II and' I`2` .at a distanc'zevv approximately twice the diameter of the main electrodenot only to prevent the evaporation thereof, but to alsoconduct land radiate energy conducted to. themrfrom the ends IIl and JI'2 and to thereby extend the useful life, of the discharge device.

Inorder to assure at least that Ythe spacing of the. secondary electrode from. the iI `remains constant, the electrode isi'rnprovedlbyhaving anotch or groove I3', e.,ga transverse groove in depth about. 1/g.the diameter of the. main electrod'e and spaced from. the end .lj I at least at the distance prescribed` for coil orfaslshown in Figure 3 andpreferably arranged. to .follow a substantially helical traversev at leastV partly circumferential about themain electrode.. The groove i3. is adapted to contact at least. part of a turn 'orrwinding of sadcoil, e, g. a turn. .I4 to, anchor the' coilv at the prescribed spacing.V The portion 0i' the coil engageable .witl'iandrecessed in said Vthe coilat tlievprescribed distance. is 'of' lessimportance than the primary improvements thereby attained. YIt is apparent Vtl'aat a groove-,engageabl'e portion of the `coil e .has a greaterarea. in thermalv contact with .the electrode than other portions of the. coil'. In such .casethe conduction of heat from, the coi-1 il to the vmainelec 0 trode is greater at the grooved area than the area near any ungrooved turn or winding. Therefore, by so grooving the main electrode and increasing its area of thermal contact with a portion of the secondary coil the heating of the main electrode is accelerated so that the shifting of the arc from the secondary electrode to the main electrode is also accelerated.

However it would not be beneficial to provide such grooves for the entire secondary electrode because upon ignition the main electrode, since it is substantially massive, would conduct heat too rapidly from the secondary electrode so that the shifting of the arc from the iii-st auxiliary to the second auxiliary would be retarded. Consequently, the improvement as shown by Figure 3 involves the grooving of the main electrode for engaging a portion oi the secondary electrode preferably at the area of the designated spacing hereinbefore set forth.

The first auxiliary electrodes i and 5 are essentially of less mass than the second auxiliary eleotrodes and, although preferably in the form of a round wire coil of tantalum or molybdenum, may be of any form as above specified with regards to the second auxiliary electrode. Tantalum or molybdenum is preferably used for the rst auxiliary electrode because these metals have better electron emission qualities when cold than does tungsten and they possess a suiciently high l melting point to exist in an unmelted form when positioned or supported on the minor portion of the main electrodes near or in contact with the inner wall of the quartz envelope i and between the inner envelope wall and at least some of the turns of the second auxiliary electrode. However, this does not preclude the use of tungsten since tungsten may be used in conjunction with a, preheating means, or tungsten and tantalum or molybdenum e. g. in the form of wires, may be employed together to form a rst auxiliary electrode coil. Other materials are better emitters than tantalum and molybdenum, e. g. thorium, alkaline earths, lead, etc., but they all evaporate rapidly at the temperatures encountered on the main electrodes. Preferably, the rst auxiliary electrodes consist of a few turns each of round wire having a diameter less than that of the second auxiliary electrodes, e. g. about one-eighth the diameter of the main electrodes, and are in electrical contact with the main electrodes as are the second auxiliary electrodes and the auxiliary electrodes may or may not be in contact with each other. Moreover, since the rst auxiliary electrodes are positioned farther from the ends of the main electrodes than are at least some of the windings of the second auxiliary electrodes, they are shielded from the temperatures of the arc between the said ends by the said second auxiliary electrodes which at least to some extent contribute to prevent the said rst auxiliary electrodes from reaching excessive temperatures.

In accordance with the present invention hereinbefore set forth, a compact type discharge device employing the above described electrode .fionizable atmosphere, a pair of tungsten rods,

'each of said rods having a length equal to at least "three times the length of the inside radius of said envelope, a major portion of each of said rods being contained and sealed by each of said extensions, a minor portion of each of said rods extending into said discharge chamber, said minor portions being spaced from each other for supporting a discharge arc, each of said minor portions comprising a main electrode, a first and second auxiliary electrode supported on said main electrode in electrical contact therewith, said second auxiliary electrode being a tungsten wire coil coaxial with said main electrode and the diameter of the wire of said coil being less than the diameter of said main electrode, said second auxiliary electrode being spaced from the free end of said main electrode, said rst auxiliary yelectrode being a wire coil formed from a metal selected from the group consisting of tungsten and tantalum, the wire of said rst auxiliary electrode being of less diameter than that of said second auxiliary electrode, said first auxiliary electrode being positioned on and coaxial with said main electrode between the inner wall of said envelope and at least part of the windings of said second electrode, a transverse groove at least partly circumferential about said main electrode and spaced from the end of said main electrode at a distance at least twice the diameter of said main electrode, a portion of said second electrode being recessed in said groove and thereby providing an increased thermal contact between said second auxiliary electrode and main electrode.

DAVID L. HILDER. References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date 2,217,438 Francis Oct. 8, 1940 2,241,345 Gustin May 6, 1941 2,241,362 Gustin May 6, 1941 2,249,094 Seitz July 15, 1941 2,249,672 Spanner July 15, 1941 2,272,467 Kern Feb. 10, 1942 2,404,953 Francis July 30, 1946 2,504,581 Power Apr. 18, 1950 FOREIGN PATENTS Number Country Date 491,176 Great Britain Aug. 29, 1938 526,064 Great Britain Sept. 10, 1940 573,141 Great Britain Oct. 8, 1945 

